Transcoding is not a “static” matter, it is dynamic because you may have in input a very wide range of content’s types and you may have to set encoding parameters accordingly (This is particularly true for user generated contents).

Not only, the elaborations that you need to do in a video project may go beyond a simple transcoding and involve a deeper capacity of analysis, handling and “filtering” of video files.

Let’s consider some examples:

1. you have input files of several resolutions and aspect ratios and you have to encode them to two target output formats (one for 16:9 and one for 4:3) . In this case you need to analyze the input file and decide what profile to apply depending by input aspect ratio.

2. now let’s suppose you want also to encode video at the target resolution only if the input has an equal or higher resolution and keep the original otherwise. Again you’d need some external logic to read the metadata of the input and setup a dedicated encoding profile.

3. sometime video needs to be filtered, scaled and filtered again. Like , for istance, deinterlacing, watermarking and denoising. You need to be able to specify a sequence of filtering and/or manipulation tasks.

4. everybody needs thumbnails generation, but it’s difficult to find a shot really representative of the video content. Grabbing shots only on scene changes may be far more efficient.

FFmpeg can satisfy these kinds of complex analysis, handling and filtering tasks even without an external logic using the embedded filtering engine ( -vf ). For very complex workflows an external controller is still necessary but filters come handy when you need to do the job straight and simple.

FFmpeg filtering is a wide topic because there are hundreds of filters and thousands of combinations. So, using the same “recipe” style of the previous articles of this series, I’ll try to solve some common problems with specific command line samples focused on filtering. Note that to simplify command lines I’ll omit the parameters dedicated to H.264 and AAc encoding. Take a look at previous articles for such informations.

1. Adaptive Resize

In FFmpeg you can use the -s switch to set the resolution of the output but this is a not flexible solution. Far more control is provided by the filter “scale”. The following command line scales the input to the desired resolution the same way as -s:

ffmpeg -i input.mp4 -vf "scale=640:360" output.mp4

But scale provides you also with a way to specifing only the vertical or horizontal resolution and calculate the other to keep the same aspect ratio of the input:

ffmpeg -i input.mp4 -vf "scale=640:-1" output.mp4

With -1 in the vertical resolution you delegate to FFmpeg the calculation of the right value to keep the same aspect ratio of input (default) or obtain the aspect radio specified with -aspect switch (if present). Unfortunately, depending by input resolution, this may end with a odd value or an even value witch is not divisable by 2 as requested by H.264. To enforce a “divisible by x” rule, you can simply use the emebedded expression evaluation engine:

ffmpeg -i input.mp4 -vf "scale=640:trunc(ow/a/2)*2" output.mp4

The expression trunc(ow/a/2)*2 as vertical resolution means: use as output height the output width (ow = in this case 640) divided for input aspect ratio and approximated to the nearest multiple of 2 (I’m sure most of you are practiced with this kind of calculation).

2. Conditional resize

Let’s go further and find a solution to the problem 2 mentioned above: how to skip resize if the input resolution is lower than the target ?

This command line uses as width the minimum between 640 and the input width (iw), and then scales the height to maintain the original aspect ratio. Notice that “,” may require to be escaped to “\,” in some shells.

With this kind of filtering you can easily setup a command line for massive batch transcoding that adapts smartly the output resolution to the target. Way to use the original resolution when lower than the target? Well, if you encode with -crf this may help you save alot of bandwidth!

3. Deinterlace

SD content is always interlaced and FullHD is very often interlaced. If you encode for the web you need to deinterlace and produce a progressive video which is also easier to compress. FFmpeg has a good deinterlacer filter named yadif (yet another deinterlacing filter) which is more efficient than the standard -deinterlace switch.

This command deinterlace the source (only if it is interlaced) and then scale down to half the horizontal and vertical resolution. In this case the sequence is mandatory: always deinterlace prior to scale!

4. Interlacing aware scaling

Sometimes, especially if you work for ipTV projects, you may need to encode interlaced (this is because legacy STBs require interlaced contents and also because interlaced may have higher temporal resolution). This is simple, just add -tff or -bff (top field first or bottom field first) in the x264 parameters. But there’s a problem: when you start from a 1080i and want to go down to an interlaced SD output (576i or 480i) you need an interlacing aware scaling because a standard scaling will break the interlacing. No fear, recently FFmpeg has introduced this option in the scale filter:

ffmpeg -i input.mp4 -vf "scale=720:576:-1" output.mp4

The third optional flag of filter is dedicated to interlace scaling. -1 means automatic detection, use 1 instead to force interlacing scaling.

5. Denoising

When seeking for an high compression ratio it is very useful to reduce the video noise of input. There are several possibilities, my preferite is the hqdn3d filter (high quality de-noising 3d filter)

The filter can denoise video using a spatial function (first two parameters set strength) and a temporal function (last two parameters). Depending by the type of source (level of motion) can be more useful the spatial or the temporal. Pay attention also to the order of the filters: deinterlace -> denoise -> scaling is usually the best.

6. Select only specific frames from input

Sometime you need to control which frames are passed to the encoding stage or more simply change the Fps. Here you find some useful usages of the select filter:

ffmpeg -i input.mp4 -vf "select=eq(pict_type,I)" output.mp4

This sample command filter out every frame that are not an I-frame. This is useful when you know the gop structure of the original and want to create in output a fast preview of the video. Specifing a frame rate for the output with -r accelerate the playback while using -vsync0 will copy the pts from input and keep the playback real-time.

Note: The previous command is similar to the input switch -skip_frame nokey ( -skip_frame bidir drops b-frames instead during deconding, useful to speedup decoding of big files in special cases).

ffmpeg -i input.mp4 -vf "select=not(mod(n,3))" output.mp4

This command selects a frame every 3, so it is possible to decimate original framerate by an integer factor N, useful for mobile low-bitrate encoding.

7. Speed-up or slow-down the video

It is also funny to play with PTS (presentation time stamps)

ffmpeg -i input.mp4 -vf "setpts=0.5*PTS" output.mp4

Use this to speed-up your video of a factor 2 (frame are dropped accordingly), or this below to slow-down:

ffmpeg -i input.mp4 -vf "setpts=2.0*PTS" output.mp4

8. Generate thumbnails on scene changes

The filter thumbnail tries to find the most representative frames in the video. Good to generate thumbnails.

A different way to achieve this is to use again select filter. The following command selects only frames that have more than 40% of changes compared to previous (and so probably are scene changes) and generates a sequence of 5 png.

The world of FFmpeg filtering is very wide and this is only a quick and “filtered” view on this world. Let me know in the comments or on twitter (@sonnati) if you need more complex filters or have problems adventuring in this fabulous world 😉

About Me

Hello, my name is Fabio Sonnati. I'm an independent ICT consultant with a master degree in Electronic Engineering.
My main expertise is the development / optimization of video delivery platforms and encoding solutions. I have contributed to several video projects on the Internet and collaborated with big names of the industry, start-ups and companies ranked in Fortune500.
This is the blog where I want to share my knowledge and best-practices about Streaming and Video Encoding. More About Me